Image Effectastic.

Because how awesome is this?

![stamen-watercolor-desaturated](https://f.cloud.github.com/assets/53660/100109/a0947cac-67ec-11e2-8c7f-4462d92fc614.png)

 * Store image effect definitions in NSUserDefaults, allowing user overrides and easier definition.
 * Image effects define an array of filters, allowing filter composition.
 * Add a "desaturate" image effect.
 * Use a new defaults key (selectedImageEffectId) since the old key was an integer.
 * Use an affine clamp before applying filters (so that gaussian blur and gloom type filters don't look awful).